Anticorrosion coating composition with improved stability, and coated substrate

ABSTRACT

The invention relates to an anticorrosion coating composition for a metal substrate, comprising at least: 
     (a) a particulate metal, 
     (b) chromic acid and/or one of its derivatives, 
     (d) a pH regulator such as zinc oxide (ZnO) or cobalt carbonate (CoCO 3 ), 
     (e) a high-boiling solvent of the polyol or glycol ether type, 
     (f) an organic cosolvent with a specified solubility parameter, 
     (g) a defoaming solvent of the mineral spirit or naphtha type, 
     (h) a system of nonionic surfactants comprising at least one water-soluble surfactant and a surfactant which is soluble in mineral spirit, and 
     (i) water. 
     It also relates to metal substrates covered with the anticorrosion coating resulting from heating after the application of the composition to the substrate.

The present invention relates to an anticorrosion coating compositionfor a metal substrate, capable of being used in particular for theprotection of screwed devices and to metal substrates covered with thiscoating.

In the field of the fight against corrosion it is known to usecompositions based on metal particles, chromic acid in solution, aglycol polymer and a wetting agent.

Thus, French Patents 2,008,770, 2,149,434 and European Patent 0,017,243,as well as French Patent Application 2,561,668, describe compositions ofthis kind and their improvements.

Industrial use of the compositions described in these documents hasshown, however, their high sensitivity to foaming. In fact, theconditions of application of these compositions involve an entry of airduring use, and this creates an air-solution emulsion, while the removalof air is prevented by the surface tension and the viscosity of thesecompositions.

A major disadvantage of an emulsion of this type is the retention of acertain quantity of composition in the threads of the screwed deviceswhich are treated, and this leads to defects in the coating. This canresult in considerable risks that the parts will be rejected aftertreatment.

The Applicant has demonstrated that the introduction of solvent from thegroup of heavy aliphatic or aromatic hydrocarbons into a composition ofthis type solves the problem of foaming (these may be, in particular,solvents of the naphtha or mineral spirit type, originating from thedistillation of oils. The defoaming solvents will be referred tohereinafter by the name of "mineral spirit" although mineral spirit isonly an individual example). Nevertheless, mineral spirit is not usuallymiscible with the compositions of the type defined above, thecompositions are unstable and it is detrimental to the formation of thefilm of anticorrosion composition at the time of coating.

The Applicant has solved this problem of the introduction of mineralspirit into the anticorrosion compositions and has developed ananticorrosion composition in which, on the one hand, mineral spirit isdispersed and, on the other hand, the wetting and film-formingproperties are maintained.

The present invention relates, therefore, to an anticorrosion coatingcomposition for a metal substrate comprising at least:

(a) a particulate metal,

(b) chromic acid and/or one of its derivatives,

(c) boric acid and/or one of its derivatives,

(d) a pH regulator, such as zinc oxide (ZnO) or cobalt carbonate(CoCO₃),

(e) a high-boiling solvent of the polyol or glycol ether type,

(f) man organic cosolvent with a specified solubility parameter,

(g) a defoaming solvent of the mineral spirit or naphtha type,

(h) a system of nonionic surfactants comprising at least onewater-soluble surfactant and a surfactant which is soluble in mineralspirit, and

(i) water.

The use of mineral spirit in an anticorrosion coating compositionenables the utilization of the compositions according to the inventionto be decisively improved when they are applied onto the metalsubstrate, as will become apparent in the examples, without compromisingthe good features of the coatings produced.

It will be possible to envisage certain alternative forms of thecompositions described below by referring particularly to theabovementioned patents and patent applications. The percentages are on aweight basis, unless indicated otherwise.

According to the invention, it is possible to use any metal or alloyhaving a negative normal electrode potential whose absolute value isgreater than that of iron. Those involved may be, in particular, zinc,aluminum, cadmium and mixtures or alloys thereof, for example withnickel, iron or cobalt. More particularly, use is made according to theinvention of zinc or of aluminum or alloys thereof, in particulate orlamellar (flake) form.

A mixture of zinc and aluminum particles may also be used according tothe invention.

The size of the metal particles is preferably chosen in a manner whichis conventional for an anticorrosion composition of this kind.

The compositions according to the invention preferably comprise fromapproximately 10 to approximately 40% of particulate metal.

According to the invention, the particulate metal may be introducedequally well in the form of a paste comprising the particulate metal anda liquid component such as mineral spirit, and in the form of dryparticles. In particular, pastes containing from approximately 85% toapproximately 92% by weight of metal and from approximately 8% toapproximately 15% by weight of mineral spirit may be used.

The mineral spirit which, as has been seen, has an "antibubbling" effectmay therefore be introduced with the particulate metal or independently,this being immaterial. It accounts for approximately 0.1 toapproximately 10% of the composition, preferably from approximately 0.5to approximately 6%.

Ordinary mineral spirit may be employed, that is to say a cutoriginating from the distillation of oils, whose boiling point isbetween 80° and 200° C., and which consists of various aliphatic andaromatic hydrocarbons (containing 15% of aromatics, for example).

Other essential constituents according to the invention are chromic andboric acids, usually employed in compositions of this type, which areuseful because of their anticorrosive effect and are introduced in aproportion of approximately 1 to approximately 12% of the composition.

Zinc oxide or cobalt carbonate are preferably used according to theinvention as a pH regulator, useful for the stability of thecomposition, the pH regulator accounting for approximately 0.5 toapproximately 4% of the composition.

Owing to its slow volatilization, the high-boiling solvent allows auniform anticorrosive coating to form after the composition has beenapplied and when it is heated. It additionally permits the conversion ofchromium(VI) to chromium(III), and other redox reactions which takeplace when the coating is formed.

According to the invention, a solvent of the polyol or glycol ethertype, such as, for example, dipropylene glycol, may be used inparticular. This high-boiling solvent accounts for approximately 7% toapproximately 30% of the weight of the composition.

Besides water, mineral spirit and the high-boiling solvent, thecompositions according to the invention comprise a cosolvent with aspecified solubility parameter, which permits the miscibility of mineralspirit with the mixture. In order to guarantee this miscibility, thesolubility parameter of the cosolvent, according to the invention, liesbetween that of mineral spirit and that of the high-boiling solvent.Preferably, a cosolvent having a solubility parameter of between 9 and12, preferably 10.8±1 is chosen, according to the invention.

The quantity of cosolvent is preferably at least equal to the quantityof mineral spirit, that is to say it lies at least between approximately0.5% and approximately 10% of the weight of the composition, andpreferably between approximately 0.5 and 6%.

By way of example of a suitable cosolvent according to the invention,use may be made of ethylglycol acetate or propylene glycol methyl etheracetate (Dowanol), although other cosolvents can be envisaged, oncondition that they are compatible with the solvent of the glycol ethertype, the mineral spirit and the chromic acid.

The simultaneous presence of water and of mineral spirit requires thepresence of a system of surfactants which enable good properties ofwetting of the components and of film formation to be ensured. Thissystem comprises, on the one hand, at least one water-soluble surfactantand, on the other hand, at least one surfactant which is soluble inmineral spirit. These are preferably nonionic surfactants.

Typically, the former, water-soluble, surfactants have an HLB valuegreater than 10, for example of the order of 12.5, while the latter,which are insoluble in water but soluble in the aromatic and/oraliphatic solvents have a HLB value below 10, for example of the orderof 8.7.

According to the invention, the system of surfactants accounts forapproximately 1% to approximately 3% by weight of the composition.

Suitable surfactants according to the invention may be chosen from theformula of the nonionics, for example alkylphenol polyethoxyethers,especially, oxyethylenated nonylphenols. By way of example, it ispossible to use surfactants from the Remcopal® (Gerland) range,especially Remcopal® 334 (soluble in solvents) and Remcopal® 349(water-soluble).

Lastly, by way of an optional constituent, it is also possible to add tothe composition other components, such as thickeners, which permitbetter application of the compositions and/or solid lubricants, whichimprove the mechanical properties of the coating produced.

A water-soluble, nonionic polymer, for example hydroxyethyl cellulose,may be employed as a thickener, although other, especiallycellulose-based, thickeners may be envisaged, on condition that they arecompatible with the presence of chromic acid.

According to the invention, approximately 0.1% to approximately 2% byweight of hydroxyethyl cellulose may be added. When other thickeners, ofdifferent molecular weight, are employed, the weight proportionindicated above varies.

PTFE or MoS₂ may be used as a solid lubricant, as in other aqueousanticorrosion compositions. According to the invention, the compositionsmay comprise up to approximately 10% of solid lubricant.

The invention relates to compositions in which all the components aremixed, as well as to compositions which are presented in the form of thevarious components packaged separately, and to compositions in the formof premixes of various components. In general, the various components ofthe compositions are mixed before being applied onto the substrate, inthe form of at least two premixes, one in an aqueous medium and one inan organic medium, as will become apparent in the examples.

The compositions according to the invention are employed in thetraditional processes for coating metal substrates, especially steel,cast iron and sintered steel substrates.

Coating of the substrate is performed by an appropriate process, forexample by dipping or spraying with a gun, and the coated substrate isthen heated to an appropriate temperature and for an appropriate timeuntil the coating hardens on the substrate, with the heating causing, inparticular, the evaporation of the solvents and the internal chemicalreactions enabling the formation of the coating to take place. Inaddition, the heating may be preceded by a draining operation, forexample by centrifuging, particularly where nuts and bolts areconcerned.

The coating may be produced in several layers; the application andhardening stages are then performed a number of times until the requiredcoating thickness is produced.

As in conventional coating processes, the substrates may be pretreated,for example by alkaline degreasing followed by rinsing and finesandblasting.

Furthermore, the coated components may form the subject of othersubsequent treatments such as lacquering, varnishing and paintapplications.

The compositions containing mineral spirit according to the inventionresult in greater stability of the product during use, particularly usein line, by virtue of the better complete control of the wettingproperties with time. In addition, they are characterized by bettersuitability for industrial use, because of the better matching of thesurface tension, because of the stability of their relative density andthe absence of foaming.

The constancy of relative density with time avoids the adjustment ofcentrifuging. In fact, at given centrifuging conditions, the constantrelative density enables the same coating thickness to be deposited allthe time.

Lastly, the distribution and the appearance of the coating which thecompositions according to the invention enable to be produced on thesubstrates to be coated in particular metal substrates incorporatingthreadings, are better, especially because of the absence of bubbles andof excessive thicknesses in the threads or on the treated articles. Themechanical performance and the corrosion resistance of the coatedsubstrates are consequently improved and losses due to rejects arereduced.

This is why the present invention also relates to the metal substratescovered with an anticorrosion coating resulting from heating acomposition according to the invention after its application onto thesaid substrate. These are particularly screws, bolts, nuts, clips,springs or components made of stamped steel, cast iron or sinteredsteel, especially components of car bodywork and of domestic electricappliances.

Other features and advantages of the present invention will becomeapparent from reading the examples hereinafter and in the attachedfigure.

EXAMPLES 1 to 4 Examples of a composition containing mineral spirit

    ______________________________________                                        EXAMPLES 1 to 4                                                               Examples of a composition containing mineral spirit                                      Ex. 1 Ex. 2     Ex. 3   Ex. 4                                      ______________________________________                                        Zinc flakes  20      20        30    17                                       Aluminum flakes                      3                                        Dipropylene glycol                                                                         10      15        20    15                                       Ethylglycol acetate                                                                        5       4         2     4                                        Mineral spirit                                                                             5       3         2     3                                        Remcopal  ® 334                                                                        1       0.7       0.5   0.7                                      Remcopal  ® 349                                                                        1.6     1         1.5   1                                        Water        51.9    50.8      35    50.8                                     Chromic acid 3       3         4     3                                        Zinc oxide   1       1         2     1                                        Boric acid   1       1         2     1                                        HEC          0.5     0.5       1     0.5                                                   100     100       100   100                                      ______________________________________                                    

The mineral spirit contains 15% of aromatics, distilled between 150° and195° C. and has a flashpoint of 35° C.

EXAMPLE 5 Foaming-defoaming test

The following measurements of relative density make it possible toappreciate the influence of the presence of mineral spirit on defoamingin a bath of anticorrosion composition.

    ______________________________________                                        initial relative                                                              density O h O h 1 min 2 h    3 h  4 h  6 h  8 h                               ______________________________________                                        A   1.35        1.26      1.28 1.29 1.29 1.30 1.31                            B   1.35        1.28      1.32 1.33 1.33 1.34 1.35                            C   1.35        1.29      1.33 1.34 1.35 1.35 1.35                            D   1.35        1.29      1.34 1.35 1.35 1.35 1.35                            ______________________________________                                         A  reference composition without mineral spirit without mechanical            defoaming;                                                                    B  reference composition without mineral spirit: mechanical defoaming;        C  composition with zinc in paste form, 2% mineral spirit;                    D  composition with zinc in powder form, 2% mineral spirit.              

The foaming is produced by deliberate stirring with a beater for oneminute. The presence of mineral spirit considerably affects thedefoaming.

EXAMPLE 6 Relative density stability test

FIG. 1 shows measurements of the relative density d of antifoamingcompositions with time (in days), at constant solids content.

The composition containing mineral spirit (O) retains a relative densitywhich is practically constant for four days. The relative density of theother two compositions (□ standard Dacromet®, results for which aregiven by way of comparison, falls during the same period.

I claim:
 1. An anticorrosion coating composition for a metal substratewhich comprises:(a) from approximately 10 to approximately 40% by weightof a particulate metal, (b) from approximately 1 to approximately 12% byweight of boric acid and hexavalent chromiun, (c) from approximately 0.5to approximately 4% by weight of a pH regulator, (d) from approximately7 to approximately 30% by weight of a high-boiling solvent of the groupconsisting of polyols and glycol ethers, (e) from at least approximately0.1 to approximately 10% by weight of an organic cosolvent with asolubility parameter of 10.8±1, (f) from approximately 0.1 toapproximately 10% by weight of mineral spirits (g) from approximately 1to approximately 3% by weight of a system of nonionic surfactantscomprising at least one water-soluble surfactant which has a HLB closeto but higher than 10 and a surfactant which is soluble in mineralspirits which has a HLB close to but lower than 10, (h) water.
 2. Thecomposition as claimed in claim 1, wherein the particulate metal ischosen from of the group consisting of zinc, aluminum, cadmium andmixtures and alloys thereof which have a negative normal electrodepotential whose absolute value is greater than that of iron.
 3. Thecomposition as claimed in claim 1, which comprises from approximately0.5 to approximately 6% by weight of mineral spirits.
 4. The compositionas claimed in claim 1, which comprises a quantity of .Iadd.said.Iaddend.organic cosolvent with a specified solubility parameter, whichis at least practically equal to the quantity of mineral spirits, on aweight basis.
 5. The composition as claimed in claim 4, wherein theorganic cosolvent has a solubility parameter lying between thesolubility parameters of .[.said.]. mineral spirits and of thehigh-boiling solvent.
 6. The composition as claimed in claim 4, whereinthe organic cosolvent is chosen from the group consisting of ethylglycolacetate and propylene glycol methyl ether acetate.
 7. The composition asclaimed in claim 1, which additionally comprises a thickening agent. 8.The composition as claimed in claim 1, which additionally comprises upto approximately 10% of solid lubricant.
 9. The composition as claimedin claim 8 wherein the solid lubricant is selected from the groupconsisting of PTFE and MoS₂.
 10. The composition as claimed in claim 1,wherein the particulate metal is introduced into the composition in theform of a paste comprising the said metal in the form of particles andmineral spirit.
 11. The composition as claimed in claim 10, wherein thepaste comprises from approximately 85 to approximately 92% by weight ofparticulate metal and from approximately 8 to approximately 15% byweight of mineral .[.spirit.]..Iadd.spirits.Iaddend..
 12. Thecomposition as claimed in claim 1, when made by a process wherein themetal is introduced into the composition in the form of dry particles.13. The composition as claimed in claim 1 in which said pH regulator isselected from the group consisting of zinc oxide and cobalt carbonate.14. The composition as claimed in claim 4 in which said cosolvent has asolubility parameter between about 9 and about
 12. 15. The compositionof claim 1 in which said cosolvent is selected from the group consistingof ethylglycol acetate and propylene glycol methyl ether acetate. 16.The composition of claim 1 in which said thickening agent compriseshydroxyethyl cellulose.
 17. The composition according to claim .[.1.]..Iadd.7 .Iaddend.wherein the hexavalent chromiun is introduced by theaddition of chromic acid.